Current limiting circuits are used to control current flow in electronic circuitry. Typically, a voltage regulator includes a current limiting circuit for overcurrent protection that prevents excessive current from flowing into a load circuit. Such a current limiting circuit includes a current sensing circuit to monitor current output from the voltage regulator.
Referring to FIG. 1, a circuit diagram illustrating an example of a conventional voltage regulator 100 is described.
The voltage regulator 100 includes an output transistor M101, resistors R101 and R102, a reference voltage generator 101, an error amplifier 102, an input terminal IN, and an output terminal OUT. The voltage regulator 100 further includes a current limiting circuit 103. The current limiting circuit 103 includes transistors M102 through M104, a resistor R103, and a constant current source 105. The transistors M101, M102, and M104 are P-channel metal-oxide semiconductor (PMOS) transistors. The transistor M103 is an N-channel metal-oxide semiconductor (NMOS) transistor.
As shown in FIG. 1, the voltage regulator 100 is a series regulator that converts an input voltage Vin into a regulated output voltage Vout through the output transistor M101, and provides an output current io to a load circuit connected therewith (not shown).
In voltage regulation, the output transistor M101 receives an output control signal at a gate and controls current flow according to the control signal. The reference voltage generator 101 outputs a given reference voltage Vref. The resistors R101 and R102, forming a voltage divider connected to the output terminal OUT, serve to output a feedback voltage Vfb corresponding to the output voltage Vout. The error amplifier 102 receives the reference voltage Vref at an inverting input and the feedback voltage Vfb at a non-inverting input. Based on the received signals, the error amplifier 102 generates the output control signal, which controls operation of the output transistor M101 so that the feedback voltage Vfb is substantially equal to the reference voltage Vref.
During the voltage regulation, the current limiting circuit 103 monitors the output current io, and upon determining that the output current io becomes excessive, limits the current flow through the output transistor M101 so that the output current io does not exceed a given maximum limit.
In the current limiting circuit 103, the transistor M102 receives the output control signal at a gate, and outputs a current proportional to the output current io according to the control signal. The resistor R103 converts the proportional current into a corresponding voltage which is indicative of the output current io.
The corresponding voltage is then applied to a gate of the transistor M103, which turns on when the applied voltage reaches a given threshold level, indicating that the output current io reaches the given maximum limit. The turn-on of the transistor M103 reduces a voltage applied to a gate of the transistor M104. This results in a current limit signal applied to the gate of the output transistor M101, limiting current flow through the output transistor M101 so as to prevent the output current io from exceeding the given maximum limit.
In the voltage regulator 100 described above, the transistor M102 and the resistor R103 form a current sensing circuit. In general, such current sensing circuit is required to be small in size, especially in the application to voltage regulators. This requirement arises from a recent trend toward smaller electronic devices, where size reduction is an important issue in developing voltage regulators having current sensing capabilities.
One approach to achieving a size reduction in a current sensing circuit is to use a small transistor device for a current sensing transistor, such as the transistor M102 in the voltage regulator 100. However, since in practice a current sensing transistor is subjected to a relatively large amount of current, this approach has a limitation in that it makes the current sensing circuit prone to failures due to overloads on the small transistor device, sacrificing stability and accuracy in current sensing performance. Thus, what is needed is a reliable, small current sensing circuit that can operate in a voltage regulator without degrading current sensing performance.